The present invention generally addresses a plastic-bound gas diffusion electrode, of the type used in fuel cells, alkali chloride electrolyzers and air cells, and in particular, an electrode of this general type which contains a manganese oxide catalyst.
Gas electrodes are a recognized means for carrying out electrochemical reactions which are to take place in contact with an electrically conductive surface and in the presence of an electrolyte while a gas is being consumed or liberated. Such electrodes generally take the form of a highly porous body (e.g., made from nickel) having pore walls which are coated with a finely dispersed catalyst consisting of a precious metal. Because such catalyst electrodes permit the free access of a gas to, and unhindered removal of a gas from the electrolyte, such catalyst electrodes can be successfully used not only for electrolytic processes, but also to operate with equal success as an electrode in a fuel cell of the oxyhydrogen type, or as an oxygen depolarizer electrode in a galvanic metal-air cell. Yet another particularly interesting industrial application for such oxygen electrodes is the electrolysis of alkali chlorides.
In view of the cost of the precious metal catalysts which are used (e.g., platinum, palladium, silver or gold), many attempts have been made in the past to replace such precious metal catalysts with various manganese oxides, which also possess good electrocatalytic properties. Various crystal species of the dioxide MnO.sub.2 --.alpha.-, .beta.- and .gamma.-MnO.sub.2, of which the .gamma.-form is the most imperfect and therefore the most active, are effective. Also found to be effective are some of the lower oxides, such as Mn.sub.3 O.sub.4 (hausmannite) or Mn.sub.2 O.sub.3, which are of even greater interest due to their increased stability.
U.S. Pat. No. 4,269,691 discloses a process for preparing electrodes with manganese oxide catalysts in which porous sintered nickel plates are first impregnated with solutions of manganese salts, such as manganese nitrate, manganese carbonate or the manganese salt of a fatty acid, and then dried and heated to about 700.degree. C., at which temperature the salt residues present in the pores undergo complete pyrolytic decomposition to lower manganese oxides.
U.S. Pat. No. 4,595,643 discloses a porous air electrode which is far less expensive because the skeleton made from sintered nickel powder is omitted. The disclosed electrode is prepared from .gamma.-MnOOH by heating this compound at temperatures between 300.degree. C. and 400.degree. C., making a paste from the manganese oxides which are formed with a PTFE dispersion, adding activated carbon and carbon black, kneading, and pressing the kneaded product into a metal net. The manganese oxide catalyst contained in this known plastic-bound electrode (which possesses particularly good activity) essentially represents a mixture of Mn.sub.5 O.sub.8 and Mn.sub.2 O.sub.3, the latter being in its stable, cubically crystallizable modification. The total composition of the catalyst approximately corresponds to a manganese oxide of the formula MnO.sub.x, where 1.45&lt;x&lt;1.60.